1. Field of the Invention
This invention broadly relates to apparatus and method of forming the same for use in the production of synthetic filaments, more especially the melt extrusion of filaments from synthetic polymers such as polyester, polyamide and the like. More particularly, the present invention relates to a filter pack, in which finely divided inert material is bound, for use in a spin pot, and also to a method of binding a finely divided inert material to form a filter pack for use in a spin pot for the melt extrusion of filaments.
The term "inert material" refers to a material that is chemically resistant to or will not contaminate the melt being extruded, that will remain solid when exposed to extrusion temperatures, and that possess sufficient mechanical strength to avoid breaking down or rapid disintegration under conditions of use. Silica sand and glass beads are preferred inert materials for use.
2. Description of the Prior Art
Conventional spin pots comprise a top cap, a filter assembly, an optional perforated breaker plate which may be recessed to form the spin pack body, a spinnerette plate provided with a plurality of extrusion orifices, and a spin pack body.
Conventional filter assemblies may be formed by a series of fine metal gauzes or screens, sintered metal discs, finely divided inert material, and the like. Certain polymers, such as polyamide, experience processing problems when screen packs or sintered metal filters are utilized. This is believed to be due to the presence of gels in the molten polymer which cannot be filtered out, as they will eventually flow through even the finest screen filter if sufficient driving force is available. It is thought that when the gel particles encounter a screen or sintered metal filter the particles divide momentarily and reform as they pass through each screen layer or around the particles in the sintered metal filter. Thus, the gel particles arrive at the discharge of the filter in much the same size and shape as they entered. These particles cause defects in the yarn called nubs, which form weak points that limit the yarn's ability to be drawn and texturized. On the other hand, it is thought that when the gel particles encounter a filter of finely divided inert material, they are broken up by the relatively sharp edges of the material and do not reform due to the random nature of the polymer passages through the filter. Polymer melt spun through such a filter generally has fewer nubs and can be readily drawn and texturized. Microscopic examination of the yarn indicates that the amount of gel is about the same for a given length of filament; however, the gel particles are much smaller when the polymer has been filtered through finely divided inert material. Synthetic yarns are in great demand, even those produced from gel-contaminated polymers, and current polymer production techniques sometimes preclude eliminating the gel at the source. Therefore, filters comprised of finely divided inert material are essential in the production of yarns from these gel-bearing polymers.
A typical filter comprising finely divided inert material may be formed by locking several layers of wire screen into a filter ring or spin pack body by swaging a peripheral aluminum wire (e.g., staking ring) into a recess in the spin pack body or filter ring. Layers of graded, finely divided inert material are then poured into the cavity of the filter ring or spin pack body one layer at a time to form the filter. Loose screens and/or distribution plates may be placed on top of the material to help hold it in place, and screens may be used to separate the layers of finely divided inert material. These filters work reasonably well as long as the layers of finely divided inert material are not disturbed. However, if the filter ring is tilted, bumped, or set down heavily, the finely divided inert material will shift to form thick and thin spots in the filter and to intermix the layers of the finely divided inert material if there are no separating screens. Further, after installation of the filter assembly in the spin pot, the spin pot itself must be installed in a spinning block and connected to a polymer source, and this is practically impossible to do without tilting or bumping the spin pot to thereby disturb and unevenly distribute the layers of finely divided inert material. This results in nonuniform flow of the material to be extruded, unequal residence times, uneven polymer distribution, and inferior performance.
Compression of the filter media (finely divided inert material) is known. See, e.g., U.S. Pat. Nos. 2,266,363 to Graves and 3,348,263 to Carter. However, as this compression is obtained by screwing or otherwise connecting the spin pot to the spin block or extrusion head, there is no provision for binding the finely divided inert material while handling the spin pot prior to its installation. Further, it is known to utilize screens which are held in place by a deformable metal ring to constrain a finely divided inert material (see U.S. Pat. No. 3,028,627 to McCormick); a problem associated with this type of constraint is the tendency of the finely divided inert material to creep out around the seal ring to plug the spinnerette orifices during operation.